The present invention relates to an optical amplifier and, more particularly, to an optical amplifier which can suitably be applied to an optical transmission system for optical communication, optical switching, optical information processing, or the like.
As an optical transmission system for transmitting a plurality of optical signals having different wavelengths, conventionally, an optical transmission system (WDM (Wavelength Division Multiplexing) system) using WDM in which a plurality of optical signals having different wavelengths are coupled to one optical fiber and transmitted is known. In this WDM system, not only one-to-one transmission but also networking is rapidly making progress.
In the WDM system, optical elements such as a WDM multiplexing/demultiplexing circuit for multiplexing/demultiplexing optical signals in accordance with their wavelengths, a multiplexing/demultiplexing circuit for multiplexing/demultiplexing light components of all wavelengths at once, and an ADM (Add-Drop Multiplexer) for extracting or inserting a specific wavelength are used. When an optical signal passes through these optical elements, an intensity loss is generated, resulting in a decrease in signal intensity.
To prevent this, an optical amplifier for directly amplifying an optical signal transmitted through an optical fiber is indispensable in the WDM system.
FIGS. 4A and 4B show the first structural example of a conventional semiconductor optical amplifier. FIG. 4A shows a section taken along a line B-B in FIG. 4B along the signal light propagation direction. FIG. 4B shows a section taken along a line A-A in FIG. 4A perpendicular to the signal light propagation direction. FIGS. 4A and 4B show an example of a conventional SOA (Semiconductor Optical Amplifier) which uses an n-InP substrate 101.
Referring to FIGS. 4A and 4B, an InGaAsP active layer 102 serving as a gain medium is formed into a stripe on the n-InP substrate 101. The InGaAsP active layer 102 is buried by a p-InP layer 103 and n-InP layer 104.
A p-InP layer 105 is formed on the InGaAsP active layer 102 and n-InP layer 104. A p-InGaAsP cap layer 106 is formed on the p-InP layer 105.
A p-side electrode 107 is formed on the p-InGaAsP cap layer 106. An n-side electrode 108 is formed on the lower surface of the n-InP substrate 101.
FIG. 5 shows the saturation characteristic of the semiconductor optical amplifier shown in FIGS. 4A and 4B. Referring to FIG. 5, when the input light intensity is low, an almost constant gain is obtained even when the input light intensity increases. However, when the input light intensity exceeds a specific value, the gain abruptly decreases.
In the WDM system, a wavelength-multiplexed signal is incident as an optical signal. The number of multiplexed wavelengths incident varies every time the signal passes through an add-drop multiplexer and the like.
Assume that an optical signal whose number of multiplexed wavelengths is m becomes incident on the semiconductor optical amplifier. In this case, when the incident light intensity of the semiconductor optical amplifier is P1 (dBm) for the total of m wavelengths, the gain of the semiconductor optical amplifier is G1 (dBm).
Assume that an optical signal is added by an add-drop multiplexer, and the number of multiplexed wavelengths increases to n. In this case, when the incident light intensity of the semiconductor optical amplifier is P2 (dBm) for the total of n wavelengths, the gain of the semiconductor optical amplifier is G2 (dBm).
As described above, when the semiconductor optical amplifier shown in FIGS. 4A and 4B is used in the WDM system, the gain of the optical signal varies depending on the number of multiplexed wavelengths.
Some conventional optical amplifiers use a method of clamping the gain to a predetermined value using oscillation to prevent the variation in gain of the optical signal depending on the number of multiplexed wavelengths.
FIGS. 6A and 6B show the second structural example of a conventional semiconductor optical amplifier. Referring to FIGS. 6A and 6B, an InGaAsP active layer 202 serving as a gain medium is formed into a stripe on an n-InP substrate 201. The InGaAsP active layer 202 is buried by a p-InP layer 203 and n-InP layer 204.
An InGaAsP separate confinement heterostructure (SCH) layer 209 is formed on the lower surface of the InGaAsP active layer 202. An InGaAsP separate confinement heterostructure (SCH) layer 210 is formed on the upper surface of the InGaAsP active layer 202. The InGaAsP separate confinement heterostructure layer 210 has a grating.
A p-InP layer 205 is formed on the InGaAsP separate confinement heterostructure layer 210 and n-InP layer 204. A p-InGaAsP cap layer 206 is formed on the p-InP layer 205.
A p-side electrode 207 is formed on the p-InGaAsP cap layer 206. An n-side electrode 208 is formed on the lower surface of the n-InP substrate 201.
In the semiconductor optical amplifier shown in FIGS. 6A and 6B, since an optical signal is reflected by the grating formed on the InGaAsP separate confinement heterostructure layer 210, forward feedback occurs. Hence, the semiconductor optical amplifier can be oscillated, like a DFB laser. However, the coupling coefficient of the grating is smaller than that of a normal DFB laser, and the oscillation threshold value is large.
In the laser oscillation state of the semiconductor optical amplifier shown in FIGS. 6A and 6B, the carrier density in the gain medium is clamped to a predetermined value. However, since the oscillation threshold value is large, the carrier density is clamped to a value larger than that of a normal DFB laser.
For this reason, in the DFB semiconductor optical amplifier having a grating in FIGS. 6A and 6B, the carrier density in the gain medium (active layer 202) is constant as far as oscillation is taking place. Since the gain is proportional to the carrier density in the gain medium, the gain can be clamped to a predetermined value.
Hence, in the above oscillation state, even when the value of the current supplied to the semiconductor optical amplifier is increased, the gain of the semiconductor optical amplifiers is kept constant, although the light intensity of the oscillation light increases. In addition, when the input signal light intensity increases, the oscillation light intensity decreases so that the total light intensity in the semiconductor optical amplifiers is kept constant. For this reason, the gain of the semiconductor optical amplifier can be kept constant without generating any variation in carrier density in the semiconductor optical amplifier.
FIG. 7 shows the saturation characteristic of the semiconductor optical amplifier shown in FIGS. 6A and 6B. Referring to FIG. 7, in the semiconductor optical amplifier shown in FIGS. 6A and 6B, even when the input light intensity of externally incident signal light varies, the gain is kept at a predetermined value Go. As shown in FIG. 7, even when the number of multiplexed wavelengths of the signal light changes from m to n, and the total input power changes from P1 to P2, the gain keeps the predetermined value Go.
In the semiconductor optical amplifier shown in FIGS. 6A and 6B, the gain decreases only when the external incident light intensity further increases, and oscillation is suppressed. Conversely, as long as oscillation is occurring in the semiconductor optical amplifier shown in FIGS. 6A and 6B, the gain can be kept constant independently of the incident light intensity or the number of multiplexed wavelengths of the incident signal.
However, when the DFB semiconductor optical amplifier shown in FIGS. 6A and 6B is used, oscillation light mixes into the same optical path as that of the signal light. Hence, a wavelength filter for removing the mixed oscillation light is necessary. In addition, in the DFB semiconductor optical amplifier shown in FIGS. 6A and 6B, the oscillation light intensity is very high. If the incident signal intensity is low, the oscillation light having almost the same intensity as that of the signal light remains even when a normal wavelength filter is used.
To solve these problems, an optical amplification apparatus has been proposed (Japanese Patent Laid-Open No. 2000-12978), in which a gain region is inserted to two arm waveguides of a symmetrical Mach-Zehnder interference circuit, a light reflection means is arranged at an input port serving as the cross port of the symmetrical Mach-Zehnder interference circuit, and a laser resonator is formed by the light reflection means and the gain region. In this optical amplification apparatus, signal light input from the port to which the light reflection means is not connected is amplified in the gain region in which the signal gain is clamped to the laser oscillation threshold state, and the amplified signal light is separated from the laser oscillation light and output from a different port.
However, in the above-described conventional semiconductor optical amplifier or optical amplification apparatus, since the gain of the signal light is clamped to a laser oscillation threshold state, the gain of the signal light cannot be adjusted.